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Root shape frequency and direction of dilaceration: a CBCT study

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Root shape frequency and direction of dilaceration: a CBCT study

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Content ROOT SHAPE FREQUENCY AND DIRECTION OF
DILACERATION: A CBCT STUDY

by
Ryan Hecht

A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(CRANIOFACIAL BIOLOGY)

March 2013

ii

Dedication

To my beautiful wife Sarah Hecht,
And to my precious daughters Alexis and Kayla Hecht

iii

Acknowledgements

I would like to give special thanks to Dr. Glenn Sameshima who has helped me
greatly through this whole process. He has given me invaluable guidance and
advice and
has
devoted
countless
hours to help me complete this research project.

iv

Table of Contents
Dedication ii
Acknowledgements iii
List of Tables v
List of Figures vi
List of Graphs vii
Abstract viii
Chapter 1: Introduction 1
Tooth root dilacerations 1
Root resorption 4
Radiological detection 4

Chapter 2: Background 8
Panoramic Radiography 8
Cone Beam Computed Tomography (CBCT) 11
History of CBCT 12
Mechanism of CBCT 13
Image Resolution 16
Radiation Dosage 17
CBCT Accuracy 18
Limitations 19
Volume Rendering 21

Chapter 3: Research Objective 22
Chapter 4: Material and Methods 23
Data Acquisition 25
Procedure 25

Chapter 5: Results 31
Statistical Data Analysis 31

Chapter 6: Discussion 39
Chapter 7: Conclusion 41
References 43

v

List of Tables
Table 1: Crosstab (Ethnicity vs. Root Shape) 32
Table 2: CBCT UR3 Dilaceration Direction Frequency 33
Table 3: CBCT UR2 Dilaceration Direction Frequency 33
Table 4: CBCT UR1 Dilaceration Direction Frequency 33
Table 5: CBCT UL1 Dilaceration Direction Frequency 34
Table 6: CBCT UL2 Dilaceration Direction Frequency 34
Table 7: CBCT UL3 Dilaceration Direction Frequency 34

vi

List of Figures
Figure 1: Lingual dilaceration 3
Figure 2: Panoramic X-ray 9
Figure 3: Mechanism of Traditional Panoramic X-ray 10
Figure 4: Non-orthogonal Panoramic X-ray 11
Figure 5: CT vs. CBCT: Fan vs. Cone 14
Figure 6: Isotropic vs. Anisotropic Voxels 16
Figure 7: Clinical application of volume rendering 21
Figure 8: Panoramic radiograph vs. CBCT 24
Figure 9: CBCT image before volume sculpting 27
Figure 10: CBCT image after volume sculpting 28
Figure 11: Maxillary right lateral incisor with distal dilaceration 29
Figure 12: Maxillary right cuspid with pointed root tip 29
Figure 13: Maxillary left lateral incisor with disto-labial dilaceration 30

vii

List of Graphs
Graph 1: Ethnicity 32
Graph 2: CBCT UL3 Direction of Dilaceration Pie Chart 35
Graph 3: CBCT UL2 Direction of Dilaceration Pie Chart 36
Graph 4: CBCT UL1 Direction of Dilaceration Pie Chart 36
Graph 5: CBCT UR1 Direction of Dilaceration Pie Chart 37
Graph 6: CBCT UR2 Direction of Dilaceration Pie Chart 37
Graph 7: CBCT UR3 Direction of Dilaceration Pie Chart 38

viii

Abstract
The primary purpose of this retrospective, exploratory study was to compare three-
dimensional (3D) diagnostics (cone beam computed tomography, CBCT) to two-
dimensional (2D) diagnostics (digital panoramic X-ray) in the assessment of root
shape and direction of dilacerations in the six maxillary anterior teeth. Panoramic
and CBCT images of 50 orthodontic patients were obtained before treatment.
Each image was evaluated to assess the shape of the roots and the direction of
dilacerations in the six maxillary anterior teeth. In calculating method error,
replicate measurements were taken 2 weeks apart on 10 subjects for all variables.
An intraclass correlation coefficient (ICC; a two-way mixed effect model) was
then calculated on the recorded measurements to determine the level of observer
reliability. ICC’s ranged from .646 to .820 for 2D and .756 to 1.000 for 3D. The
frequency of root shape was significantly different between 2D and 3D images by
Kappa correlation coefficients. The majority of root dilacerations occurred
directly distal or distal lingual. Additionally, nearly 25% of the canines' apices
were not identifiable in 2D images, but were identifiable in 3D images. There
were no significant differences recorded between the two ethnic groups by chi-
square analysis. For all statistical tests, significance was established at α = .05.
With CBCT images root shapes could be assessed and the direction of
dilacerations evaluated. With panoramic images this assessment was not always
possible. Sometimes the roots of teeth would be impossible to evaluate from

ix

panoramic x-rays because of overlying structures and artifacts. CBCT was
effective for detecting root shapes and dilacerations in vivo and allowed three-
dimensional evaluation of dental roots without overlapping images.

1

Chapter
1:
Introduction

Tooth root dilacerations

It is not uncommon for the roots of teeth to form with an angulation, sharp bend,
or a curve. Tomes, in 1848, called such curvatures “dilacerations” (Jafarzadeh and
Abbott, 2007). Dilaceration can be defined as a deviation of the apical part of the
root by 20 degree or more (Figure 1). Although any tooth may be affected, the
most frequently involved teeth are the permanent maxillary incisors, followed by
the mandibular anterior dentition (Hamasha et al., 2002). The literature reports a
wide range of dilaceration prevalence, with the frequencies ranging from 0.32% to
98% of teeth (Malcic et al., 2006; Thongudomporn and Freer 1998). Chohayeb
found that for maxillary lateral incisors, distolabial dilaceration was by far the
most common category, containing 52 percent of the lateral incisors (Chohayeb,
1983).
The most widely accepted cause of dilacerations is mechanical trauma to the
primary predecessor tooth, which results in an altered root form for the developing
succedaneous permanent tooth (Kilpatrick, 1991). The calcified portion of the
permanent tooth germ is displaced in such a way that the remainder of the
permanent tooth germ forms at an angle to it (Von Gool, 1973; Kolokithas and
Karakasis, 1979; Kearns, 1998). Although the prevalence of traumatic injuries to
the primary dentition ranges from 11%–30%, the incidence of dilacerated

2

permanent teeth is very low and disproportionate to the high prevalence of trauma
(Smith and Winter, 1981). Consequently, traumatic injuries to the primary
dentition are unlikely to account for all cases of dilaceration and especially those
of primary teeth themselves (Maragakis, 1995). Although the damage frequently
follows avulsion or intrusion of the overlying primary predecessor, an event that
normally occurs before 4 years of age (Andreasen et al., 1971; Neville et al.,
2002), some reports (Chadwick and Millett, 1995; Feldman, 1984) have
questioned the etiology of dilaceration and do not support the belief that trauma is
the major etiologic factor. Some researchers support this view because most
dilacerated teeth are found in posterior teeth, and these are not prone to direct
trauma (Yassin, 1999).
Most cases are the result of other causes that result in continued root formation
during a curved or tortuous path of eruption (Valladares, 2010). These other
causes include scar formation, developmental anomaly of the primary tooth germ,
facial clefting (Gorlin and Goldman, 1970), advanced root canal infections (Kalra
et al., 2000), ectopic development of the tooth germ and lack of space, the effect
of anatomic structures (for example, the cortical bone of the maxillary sinus, the
mandibular canal, or the nasal fossa, which might deflect the epithelial diaphragm)
(Walton and Torabinejad, 2002), the presence of an adjacent cyst, tumor, or
odontogenic hamartoma (for example, odontoma and supernumerary tooth)
(Neville et al., 2002; Atwan et al., 2000; Yeung et al., 2003; Dayi et al., 1997),

3

orotracheal intubation and laryngoscopy (Kearns, 1998; Angelos et al., 1989),
mechanical interference with eruption (for example, from an ankylosed primary
tooth that does not resorb) (Proffit et al., 2000), tooth transplantation (Monsour
and Adkins, 1984), extraction of primary teeth (Matsuoka, 2000), and hereditary
factors (Regezi et al., 2003; Lin et al., 1982; Witkop and Jaspers, 1982). Another
cause is therapeutic irradiation of the area. The cause in some cases is idiopathic.
Dilacerations are known to be associated with increased risk of external apical root
resorption (Topouzelis, 2010).

Figure 1 Maxillary central incisor demonstrating lingual dilaceration that is
impossible to view with conventional radiographs.

4

Root
Resorption

Root resorption in orthodontics has been described as an idiopathic and
unpredictable adverse effect of orthodontic treatment (Brezniak and Wasserstein,
1993). While extensive post-orthodontic root resorption compromises the benefits
of an otherwise successful orthodontic outcome, most root loss resulting from
orthodontic treatment does not decrease the longevity or the functional capacity of
the involved teeth (Pizzo et al., 2007). It commonly manifests as apical root
shortening or surface resorption (Al-Qawasmi et al., 2003). Histological studies
have reported high incidences while clinical studies have revealed a more varied
incidence (Abuabara, 2007). Although most root resorption studies attempt to
investigate the etiologic factors and predictability of this phenomenon, its origins
remain obscure (Weltman, 2010). Individual susceptibility, hereditary
predisposition, systemic, local and anatomic factors associated with orthodontic
treatment are commonly cited components (Zahrowski and Jeske, 2011).
Radiologic
detection

Numerous reports have documented root resorption using radiographs (Hollender
et al., 1980; Lundgren, 1996; Borg, 1998). This is clinically relevant in detecting
root shortening before, during or after orthodontic treatment. However, root
resorption seen on radiographs could often only detect root shortening. Surface
resorption could only be detected if it were mesio-distally placed, facing in direct
right angles to the focal beam of the X-rays or if resorption has progressed to a

5

severe or advanced stage (Lydiatt et al., 1989; Remington et al., 1989;
Costopoulos and Nanda, 1996). The common radiographs requested for diagnosis
are panoramic, periapical, and lateral cephalometric radiographs (McNab et al.,
2000).
The panoramic radiograph is a sectional radiograph and only structures that are
within the section will be captured on the film (Whaites, 1996). This in-focus
section or focal trough is approximately the shape as the dental arch and is created
by a narrow slit of X-ray beam aimed upwards at approximately 8° to the normal
(Leach et al., 1989). Relative positioning of the dental arch in this pre-determined
focal trough may lead to images that are foreshortened, magnified, and/ or out of
focus. In addition, normal anatomical structures can appear as radiolucent or
radiopaque shadows superimposed over the teeth as either real or actual shadows
or as a ghost or artifact of which can degrade the quality of the final image. In
orthodontic cases, in markedly class II or III patients, or patients with excessively
proclined or retroclined teeth, it may not be feasible to position the upper and
lower labial segments or the maligned teeth within the focal trough (Lund et al.,
2012). Roots may hence be magnified or foreshortened. Sameshima and Asgarifar
compared the use of periapical radiographs and panoramic radiographs to assess
morphology and quantify the amount of root shortening present in a human sample
(Sameshima and Asgarifar, 2001). They found that the description of root
morphology differed greatly between the two detecting modes. It was found that

6

panoramic radiographs overestimated the amount of root loss by 20% or more
when compared with periapical radiographs. This could easily be explained by the
relative position of the focal trough to the dental arch during imaging. The narrow
focal trough in the anterior portion of the maxilla presents particularly as a
problem in patients with maligned incisors. The apices are often misdiagnosed as
root resorption from root foreshortening (Leach et al., 1989).
The lateral cephalogram provides an accurate and reproducible view of the length
of the upper incisors. However, this is likely to be subjected to a 5–12%
enlargement factor as a result of the radiographic set-up. In addition, overlapping
of the left and right sides may make the image unclear (Leach et al., 1989).
Digital radiography has demonstrated a similar degree of sensitivity to film-based
radiography in the detection of resorption, but with a lower radiation dose (Heo,
2001; Kravitz, 1992) 43-46]. However the geometric relationship of the digital
receptor, tooth, and X-ray beam is just as important as in conventional radiography
if geometric distortion is to be avoided (Leach et al., 1989).
Certain root shapes and dilacerations are well known to be associated with
increased risk of external apical root resorption, however the limitations of using
panoramic radiographs can prevent assessment of root shape (Makedonas et al.,
2012). Cone beam computed tomography (CBCT) allows the hard tissues of the
maxillofacial region to be assessed in three dimensions (Scarfe et al., 2006). With

7

CBCT, overlapping images can be removed to provide an accurate assessment of
root shape and dilacerations (Patel et al., 2007). Only recently has the resolution
of CBCT improved enough to make visualization of the root apex consistently
viewable. The smaller the field of view (FOV), the better the spatial resolution
and the smaller the voxel size. It is conceivable that root shape looks different in
three dimensions; in particular root dilacerations may be more evident.

8

Chapter
2:
Background

Panoramic Radiography
Until recently, two dimensional (2D) radiographic imaging has been the
standard of care to help orthodontists evaluate the dentition, skeleton, and soft
tissues of interest for proper diagnosis, treatment planning and evaluation of
growth and development. These radiographs commonly include a lateral
cephalogram and a panorex. Panoramic radiography was developed in 1948 by
Paatero (Steiner, 1959). It has traditionally been the gold standard for care due to
the advantages of low patient radiation dose, convenience, ease and speed of the
procedure. Panoramic radiography is often used before, during and after
orthodontic treatment to assess root parallelism, mesiodistal tooth angulation, and
root resorption (Macri and Athanasiou, 1997). However disadvantages of
panoramic radiography include lack of fine detail, magnification, and distortion.
The distortion is created by the angle between inclined teeth and is the result of
vertical and horizontal distortions. Assessing mesiodistal angulations of teeth and
bone should be done so with caution due to this inaccuracy (Baumrind and Frantz,
1971; Carlson, 1967). Research done by McKee et al. demonstrated mesiodistal
angulation measurements from panoramic radiography were statistically different
from true mesiodistal angulations. The largest amount of angulation distortion can
be found in the canine premolar region of both arches. The ABO does not take off

9

points for any roots in the canine region due to this well-known inherent distortion
(Nett BC, and Huang, 2005).
A panoramic radiograph is created by curved-surface tomography in which a
narrow beam of radiation is rotated in the horizontal plane around an invisible
pivot point positioned intraorally (Figure 2).

Figure 2 Panoramic radiograph produced using curved-surface tomography. The
patient remains stationary while x-ray source and film move in opposite directions
in a fixed relationship through one or a series of rotation points.

10

Figure 3 Mechanism of image formation with panoramic radiographs. The focal
trough or “image layer”, is the plane that is not blurred on the radiograph.

The digital sensor and tubehead travel in opposite directions around the patient.
The center of rotation changes as the sensor and tubehead rotate, which allows the
image layer to conform to the elliptical shape of the dental arches. This results in
the focal trough or “image layer”, where the plane is not blurred on the
radiographic image (Figure 3). Because panoramic radiographs are projection
images of three-dimensional structures on two-dimensional planes, the three-
dimensional relationships between different structures are not truly revealed
(McDavid et al., 1986) (Figure 4).

11

Figure 4 The X-ray beam from a panoramic machine is not always orthogonal to
the target tooth. Non-orthogonal X-rays always cause distortion in the images.

Cone Beam Computed Tomography (CBCT)
Just as three-dimensional imaging in the medical field has revolutionized medical
diagnosis and the delivery of treatment since the introduction of computed
tomography (CT) in 1971 by Dr. Hounsfield in England, the CBCT has opened up
a new horizon for three-dimensional diagnosis and treatment planning in dentistry,
particularly in orthodontics where shape, form, structure, and position are of
critical importance. Applying this new technology, we can now visualize each
whole tooth in all three planes of space.

12

History of CBCT
Since Broadbent introduced cephalometric radiology to orthodontics in 1931,
imaging has been a crucial adjunct to orthodontic diagnosis and treatment
planning. Orthodontists rely on these cephalometric radiographs to derive angular
and linear measurements from anatomical landmarks. Current radiographs, such as
panoramic radiographs and lateral cephalograms, are limited to a two-dimensional
(2D) projection. These 2D diagnostic radiographic images are inherently subject
to magnification, distortion, superimposition, and misrepresentation of structures.
Since, the development of the first CT in 1967 by an engineer named, Sir. Godfrey
Hounsfield, CT technology has rapidly evolved. The first generation of CT
scanners used a single detector element to capture a beam of X-rays. Both the
detector and source rotated one degree at a time, a design known as the "translate-
rotate" or "pencil-beam" scanner. In 1975, a second generation of CT known as
"hybrid" CT was introduced. These machines used more than one detector and a
small fan-beam X-ray, as opposed to a pencil-beam. In 1976, a third generation
CT scanner was introduced. These scanners used a large, arc-shaped detector that
acquires an entire projection without the need for translation. This rotate-only
design utilizes the power of the X-ray tube much more efficiently than the
previous generations. A fourth generation scanner shortly followed, replacing the
arc-shaped detector with an entire circle of detectors. In this design the X- ray
tube rotates around the patient, while the detector stays stationary.

13

The introduction of cone beam computed tomography (CBCT) specifically
dedicated to imaging the maxillofacial region heralds a true paradigm shift from a
2D approach to a 3D approach to data acquisition and image reconstruction
(Scarfe and Farman, 2008). CBCT was initially developed for angiography (Rob,
1982), but other applications have included airway analysis (Weissheimer et al.,
2012). CBCT was originally developed as an alternative to conventional CT to
provide a more rapid acquisition of an image by using a larger field of view
(FOV), and a comparatively less expensive x-ray detector. Through the use of a
cone shaped x-ray beam, the CBCT produces shorter radiation exposure, faster x-
ray acquisition time, reduced image distortion due to patient movements, and
increased x-ray efficiency. The use of a larger FOV increases the amount of x-ray
scatter. As CBCT FOV becomes larger, the image quality related to noise and
contrast resolution is limited.
Mechanism of CBCT
Typically, CBCT machines scan patients in one of three positions: sitting,
standing, or supine. Unlike having the patient upright, a patient supine during a
scan can alter the 3D soft tissue profile, thus it is important to look at additional
photographs to accurately evaluate the soft tissue.
CBCT imaging is accomplished by using a rotating gantry to which an x-ray
source and detector are fixed. A divergent cone-shaped source of ionizing

14

radiation is directed throughout the middle area of interest onto an x-ray detector
on the opposite side. To acquire an image, a single rotation of the x-ray source and
detector rotates around a fixed fulcrum within the center of interest (Figure 5).

Figure 5 Traditional CT vs. CBCT (A) CBCT uses a “cone of x-rays and only
needs a single pass around the subject. (B) Traditional CT uses a “Fan” of x-rays
with multiple helical passes around the subject.

During the rotation, multiple sequential planar projection images of the FOV are
acquired in a complete or sometimes partial arc. This procedure varies from a

15

traditional medical CT, which uses a fan-shaped x-ray beam in a helical
progression to acquire individual image slices of the FOV. A computer stacks the
acquired slices to obtain a 3D representation. Each slice requires a separate scan
and separate 2D reconstruction. Because CBCT exposure incorporates the entire
FOV, only one rotational sequence of the gantry is necessary to acquire enough
data for image reconstruction.
The resolution of CBCT imaging is gauged by the volume elements or voxels
produced from the 3D volumetric data. The voxel dimension depends on the pixel
size on the detector. The detector typically detects pixels that range from 0.09 mm
to 0.4 mm. Therefore, the detector determines the final resolution and clarity of
the 3D volumetric image. Because CBCT raw data is obtained in one rotation of
the x-ray source, CBCT voxels are isotropic, which means that all three
dimensions are of equal length. Traditional CT scanners rotate multiple times
around the object to produce an image. To create CT voxels, the computer must
combine all the obtained slices, and construct a z dimension depending on slice
thickness. These resulting compositional voxels are anistropic, which is of unequal
dimensions. The voxel shape is columnar and has an equal x and y dimension, but
an unequal z dimension (Figure 6). Once the patient has been scanned with a
CBCT scanner, data must be processed to create a 3 dimensional volume. This
process of reconstruction combines a number of individual base projections, each
frame resembling a lateral cephalogram that contains more than one million pixels.

16

Each pixel is assigned 12 or 16 bits of data. A computer processes the multiple
base projections to reconstruct the desired 3D volume. 3D image processing time
varies based on voxel size, FOV, number of projections, and computer processing
speed.

Figure 6 Comparison of volume data sets obtained isotropically (left) and
anisotropically (right). Because CBCT data acquisition depends on the pixel size
of the area detector and not on the acquisition of groups of rows with sequential
translational motion, the compositional voxels are equal in all three dimensions,
rather than columnar with height being different from the width and depth
dimensions. [16]

Image Resolution: Pixel and Voxel Size
A pixel represents the smallest sampled 2D element in an image. The image has
dimensions along two axes in millimeters, dictating in-plane spatial resolution. A

17

voxel is the volume element, defined in 3D space. Its dimensions are given by the
pixel, together with the thickness of the slice (the measurement along the third
axis). Resolution, which determines the ability to distinguish structures as separate
and distinct from each other, is inherently related to the acquired voxel volume.
The FOV, acquisition matrix, and the slice thickness determine voxel volume. The
pixel size (FOV/matrix) determines the in-plane resolution. Reducing the FOV,
increasing the matrix number, or reducing the slice thickness results in an image
with reduced voxel volume. The smaller voxels produce images with a higher
resolution, but a lower signal-to-noise ratio (SNR), which may cause the image to
look grainy (Dawood, 2009).
CBCT Radiation Dosage
Doses can be as low as 40 mSv to 50 mSv, depending on the type and model of
CBCT scanner and FOV selected (Schulz and Thurmann, 2004).
Comparing these doses with multiples of a single panoramic dose or background
equivalent radiation dose, CBCT provides an equivalent patient radiation dose of 5
to 74 times that of a single film based panoramic x-ray, or 3 to 48 days of
background radiation (Scaf and Mosier, 1997). The use of a thyroid collar and
chin position can substantially reduce the dose by up to 40%. In comparison to a
traditional CT, CBCT radiation exposure of the maxillofacial region is 76.25% to
98.5% less (Schulze and Thurmann, 2004).
Doses can be as low as 40 mSv to 50 mSv, values (Mah, 2004) in a range similar

18

to that of conventional dental radiographic examinations. In comparison, the
effective radiation dose from a panoramic examination is in the range of 2.9 to
9.6 mSv and that from a complete mouth series ranges from 33 to 100 mSv
(Lascala, 2004).

CBCT Accuracy
Traditional orthodontic analysis typically requires a 2D lateral cephalogram and
panoramic x-ray of the skull, which can be highly inaccurate due to magnification,
anatomic superimposition, beam projection angle, and patient position (Hilgers et
al., 2005). These distortions were unavoidable, until the advent of CBCT imaging.
Unlike 2D radiographs, cephalometric radiographs constructed from CBCT scans
can use the right or left half of the skull, thereby overcoming the superimposition
of the ramus, body, molar and condyles. Also, in 2005, Hutchinson showed that
linear and angular dimensions are more accurate using a CBCT derived panoramic
radiograph, compared to traditional radiographs (Hutchinson, 2005). Furthermore,
in a CBCT imaging, because the x-ray beams emitted are almost parallel to one
another and the raw data is obtained in one rotation, the voxels are isotropic
eliminating the magnification and distortion of the image. After the CBCT scan is
complete, the computer software processes the data, and the resulting image has a
1-to-1-measurement ratio to the original object. To ensure that error is kept to a
minimum and that other operational systems are functioning correctly, a water

19

phantom is used to calibrate the 3D scanner (Mah, 2004). In 2005, Hilgers studied
the accuracy of CBCT in the TMJ. He found that CBCT accurately and precisely
depicts the TMJ complex in a 3D model. The measurements were reproducible
and significantly more accurate than those made with conventional cephalograms
in all 3 dimensional planes (Hilgers et al., 2005). In 2004, Lascala studied the
accuracy of linear measurements obtained by cone beam computed tomography.
The group studied 13 measurements in 8 dried skulls and recorded measurements
with an electronic caliper. The skulls were then scanned and analyzed with a
NewTom QR-DVT 9000 (Lascala, 2004). The radiographic distance
measurements of the same dry skull were then compared. They concluded that the
CBCT image underestimates the real distances between skull sites. Differences
were only significant for the skull base, and therefore, it is reliable for linear
evaluation measurements of other structures more closely associated with
maxillofacial imaging. CBCT scans are highly accurate and provide a three-
dimensional visualization of the tooth root morphology, free of the previous
projection and superimposition limitations of two-dimensional radiographs.
Limitations of CBCT
CBCT image artifacts arise because of the inherent polychromatic nature of the x-
ray beam. The x-ray source produces a heterogeneous mix of low and high energy
photons, which passes through an object. The low energy photons are absorbed
more than the higher energy photons, resulting in a richer high-energy photon

20

penetrating beam or beam hardening. Beam hardening manifests as a cupping
artifact, or streaks and dark bands. Cupping artifacts occur when x-rays passing
through the center of an object become harder than x-rays passing through the
edges of an object. Since the x-ray beam becomes harder in the center, the image
is processed incorrectly and produces the artifact. Patient motion can cause
blurriness of the reconstructed 3D image. Placing a head restraint on the patient
before the scan is taken can minimize this type of error.
Undersampling can also occur if too few basis projections are used in the
reconstruction. A reduced number of projections can lead to blurriness and a
noisier 3D image. Resolution of fine detail will be affected.
Scanner related artifacts can manifest as a circular or ring shaped artifacts. These
artifacts are caused by poor scanner detection or improper scanner calibration.
These problems will consistently result in circular artifacts.
CBCT image noise is mostly caused by scattered radiation. Since CBCT uses a
cone shaped x-ray beam, scattered radiation is unavoidable. This scattered
radiation becomes omnidirectional and is recorded by the CBCT detector. This
nonlinear attenuation contributes to the overall image noise (Wriedt, 2012).
A cone beam effect can be a source of artifacts. Since the x-ray beam diverges and
the beam rotates around the patient, the amount of information for peripheral
structures is reduced. The outer rows of the detector record less than the inner

21

rows, resulting in image distortion, streaking, and greater peripheral noise.
Volume Rendering
Volume rendering of the CBCT image creates a three-dimensional model using no
voxel threshold for data exclusion (Figure 7). The entire volume is always
loaded, but tissues are grouped interactively by voxel intensity, and each group
can be assigned a color and transparency value before projecting the volume onto
the viewing monitor. The operator can rotate the volume-rendering model and
change the opacity levels, thus providing the sense of peeling away tissues layer
by layer. Volume rendering is a good way to understand the anatomic
relationships between structures visually (Liu, 2010).

Figure 7 Clinical application of volume rendering. The tissues with the
lowest attenuation values get the highest transparency value.

22

Chapter
3:
Research
Objective

The primary purpose of this retrospective, exploratory study was to evaluate the
following research questions:
1) To compare three-dimensional (3D) diagnostics (cone beam computed
tomography, CBCT) to two-dimensional (2D) diagnostics (panoramic X-ray)
to determine how root shape and direction of dilacerations in the maxillary
anterior dentition.
2) Is the frequency of occurrence of the five root shape categories different when
viewing individual teeth in 2D (Panoramic) or 3D (Limited Field CBCT)?
3) Which direction is the most frequent in dilacerated roots when viewing in 3D?
4) Is there any difference in root shape frequency between Caucasian and
Hispanic patients?

23

Chapter
4:
Materials
and
Methods

The study sample was fifty orthodontic patients from the University of Southern
California graduate orthodontic clinic with complete dentitions, no previous
orthodontic treatment, and no oral habits or history of trauma to the dentition.
There were no restrictions on type of malocclusion, sex, ethnicity, crowding, or
other common dental and orthodontic measurements. For each patient both digital
panoramic and limited field CBCT images taken on the Kodak 9000 were obtained
for each subject. The University of Southern California Institutional Review
Board approved the study protocol.
Each panoramic and CBCT image was evaluated at different time points to assess
the shape of the roots and the direction of dilacerations in the six maxillary
anterior teeth. The shapes of the tooth roots were categorized into one of the
following categories: normal, pointed, blunt, pipette, or dilacerated. The direction
of tooth root dilaceration was characterized in all three dimensions (facial, palatal,
mesial, or distal).

24

Panoramic
image
of
patient
E.H.

CBCT
image
of
patient
E.H.

Figure 8 Panoramic radiograph vs. CBCT

25

Data acquisition
Each patient was scanned between 2010-2012 using a Kodak 9000 scanner
[KODAK 9000 3DH (Carestream Health, Inc.)]. The CBCT images were taken
with an isotropic voxel size of 76mm, field of view 50637mm, tube potential
85kV, and tube current 2 mA. The panoramic images were taken at average
exposure times of: Child 64kv, 10ma, 10.8 sec; Small adult: 68kv, 8ma, 13.4 sec;
Medium adult: 70kv, 10ma, 14.3 sec; Large adult: 74kv, 10ma, 15.1 sec. The
patient’s ages ranged from 13 to 41 years old, with an average age of 22 years old.

Procedure
Dolphin imaging tools were utilized to manipulate the 3D renderings so the roots
of the teeth could be completely visualized. The 3D renderings were oriented by
visual inspection to a natural head position. The first step was to remove
overlaying structures such as tissue, bone, and other teeth so that each individual
tooth root could be completely analyzed using the volume sculpting, transparency,
and segmentation tools. Also, with rotation of the image in all three planes of
space, the root shape could be characterized and direction of dilaceration
determined.
To view the roots of the teeth the translucent hard tissue density segmentation was
adjusted. The steps to accomplish this was to:

26

1) Select the Hard Tissue radio button.
2) Select the Translucent radio button for a translucent view of the patient's
hard tissue.
3) Adjust the Trans slider to adjust the transparency of the image.
4) Adjust the Seg slider to set the segmentation for the hard tissue view.
The translucent and segmentation adjustments can enable the visualization of
structures that might otherwise be difficult to see.
The volume-sculpting tool was used to remove parts of the volume such as
overlaying images of other teeth, bone, and tissue. The images could be viewed
from any angle by rotating the 3D volume.

27

Figure
9
CBCT
image
before
volume
sculpting

28

Figure
10

CBCT
image
after
selective
application
of
volume
sculpting

29

Figure
11

Maxillary
right
lateral
incisor
with
distal
dilaceration

Figure
12

Maxillary
right
cuspid
with
pointed
root
tip

30

Figure
13
Maxillary
left
lateral
incisor
with
disto-‐labial
dilaceration

31

Chapter
5:
Results

Statistical
data
analysis

All data were entered into Excel:mac 2011 (Microsoft, Redmond, Wash). The
statistical analyses were carried out with SPSS (Version 12.0, SPSS, Chicago, Ill).
Pearson correlation coefficients were performed to determine the reliability
between the first and second measurements. In calculating method error, replicate
measurements were taken 2 weeks apart on 10 subjects for all variables. An
intraclass correlation coefficient (ICC; a two-way mixed effect model) was then
calculated on the recorded measurements to determine the level of observer
reliability. ICC’s ranged from .646 to .820 for 2D and .756 to 1.000 for 3D. The
frequency of root shape was significantly different between 2D and 3D images by
Kappa correlation coefficients. The majority of root dilacerations occurred
directly distal or distal lingual. Additionally, nearly 25% of the canines' apices
were not identifiable in 2D images, but were identifiable in 3D images. There
were no significant differences recorded between the two ethnic groups by chi-
square analysis. For all statistical tests, significance was established at α = .05.

32

Tables
and
charts

Crosstab (Ethnicity vs. Root Shape)
C=Caucasian, H=Hispanic, B=Blunt,
N=Normal, P=Pointed, X=Missing

Count

UR3 Pan
Total

B N P X
Ethnicity C 2 11 2 1 16
H 3 21 3 1 28
Total 5 32 5 2 44

Chi-‐Square
Test:

P=.965
(not
significant)

56%

31%

11%

2%

Ethnicity

H
-‐
Hispanic

C
-‐
Caucasian

A
-‐
Asian

I
-‐
Indian

33

Direction of Dilaceration Frequency Tables
D=Distal, DF=Distal-Facial, DL=Distal-lingual, M=Mesial, ML=Mesial-lingual
L=Lingual, N=Normal, X=Missing
CBCT UR3

Frequency Percent
Valid
Percent
Cumulative
Percent
Valid D 6 12.0 12.0 12.0
DL 1 2.0 2.0 14.0
M 4 8.0 8.0 22.0
N 38 76.0 76.0 98.0
X 1 2.0 2.0 100.0
Total 50 100.0 100.0

CBCT UR2

Frequency Percent
Valid
Percent
Cumulative
Percent
Valid D 18 36.0 36.0 36.0
DL 1 2.0 2.0 38.0
M 3 6.0 6.0 44.0
N 27 54.0 54.0 98.0
X 1 2.0 2.0 100.0
Total 50 100.0 100.0

CBCT UR1

Frequency Percent
Valid
Percent
Cumulative
Percent
Valid D 1 2.0 2.0 2.0
DF 1 2.0 2.0 4.0
L 1 2.0 2.0 6.0
N 46 92.0 92.0 98.0
X 1 2.0 2.0 100.0
Total 50 100.0 100.0

34

CBCT UL1

Frequency Percent Valid Percent
Cumulative
Percent
Valid D 2 4.0 4.0 4.0
L 1 2.0 2.0 6.0
M 1 2.0 2.0 8.0
ML 1 2.0 2.0 10.0
N 45 90.0 90.0 100.0
Total 50 100.0 100.0

CBCT UL2

Frequency Percent Valid Percent
Cumulative
Percent
Valid D 15 30.0 30.0 30.0
DL 2 4.0 4.0 34.0
ML 1 2.0 2.0 36.0
N 31 62.0 62.0 98.0
X 1 2.0 2.0 100.0
Total 50 100.0 100.0

CBCT UL3

Frequency Percent
Valid
Percent
Cumulative
Percent
Valid D 9 18.0 18.0 18.0
DF 1 2.0 2.0 20.0
DL 2 4.0 4.0 24.0
F 1 2.0 2.0 26.0
M 2 4.0 4.0 30.0
N 31 62.0 62.0 92.0
X 4 8.0 8.0 100.0
Total 50 100.0 100.0

35

Direction of Dilaceration Frequency Pie Charts
D=Distal, DF=Distal-Facial, DL=Distal-lingual, M=Mesial, ML=Mesial-lingual
L=Lingual, N=Normal, X=Missing

36

37

38

39

Chapter
6:
Discussion

1.) To compare three-dimensional (3D) diagnostics (cone beam computed
tomography, CBCT) to two-dimensional (2D) diagnostics (panoramic X-
ray) to determine how root shape and direction of dilacerations in the
maxillary anterior dentition.
With CBCT images the root shapes could be characterized as one of the five
categories of root shape and the direction of dilacerations could be specified.
With panoramic images this assessment was not always possible. The method
error differences between 2D and 3D indicate that CBCT 3D diagnostics are
superior for determining root shape and the direction of dilacerations.

2.) Is the frequency of occurrence of the five root shape categories different
when viewing individual teeth in 2D (Panoramic) or 3D (Limited Field
CBCT)?
With CBCT images there was far less method error in characterizing the root
shape. This indicates that with CBCT 3D diagnostics there is less variation in
determining the root shape compared to panoramic images.

3.) Which direction is the most frequent in dilacerated roots when viewing in
3D?

40

In 3D the most frequent direction of dilaceration of the tooth root is to the
distal.

4.) Is there any difference in root shape frequency between Caucasian and
Hispanic patients?
There was no statistical difference in root shape frequency between the
Caucasian and Hispanic orthodontic patients of this study.

Future studies of these patients after orthodontic treatment has been competed
should measure the apical displacement of the tooth root apex. This
information will be beneficial to establish what relationship exists between the
distance of apical root displacement and the increased risk of external apical
root resorption (EARR). Other studies have shown that apical root
displacement could be a risk factor for EARR.

41

Chapter
7:
Conclusions

The use of CBCT imaging in dental practice is increasing as a tool for diagnosis,
detection, and treatment planning. Sometimes the roots of teeth would be
impossible to evaluate from two-dimensional x-rays because of overlying
structures, artifacts, and image distortion. CBCT imaging was effective in this
study for detecting root shapes and dilacerations in vivo and allowed for the three-
dimensional evaluation of tooth root morphology.
From this study the following conclusions were:
1) In 3D more teeth look pointed than in 2D, and fewer teeth look blunted. Better
resolution around the root apex would improve this study.
2) There were no differences in root shape frequency between Hispanic and
Caucasian patients – in contrast to previous studies in 2D.
3) There were notable occurrences of dilaceration of maxillary teeth where the
direction of curvature was lingual to the rest of the tooth rendering it not
visible in 2D.
In conclusion, small volume CBCT may be justified as a supplement to a routine
panoramic X-ray in the following cases: when root resorption of teeth is
suspected, and/or when the tooth root apex is not clearly discernible in the
panoramic X-ray, implying dilaceration of the tooth root.
The amount of radiation from a small limited field CBCT scan compared to the
standard combination of periapical, bitewing, lateral cephalometric, and

42

panoramic radiographs that an orthodontic patient would require for proper
diagnosis and treatment planning are comparable. Compared to large field CBCT,
the small field CBCT radiation dose is much smaller. Therefore, there is a good
clinical indication for taking maxillary limited field CBCT image for orthodontic
patients.
Future studies should be done to again categorize the root shape and the direction
of dilacerations after orthodontic treatment has been completed on these patients.
This information would be valuable in determining if certain root shapes or
directions of dilaceration correlate with an increased risk of external apical root
resorption (EARR). It is conceivable that when the tooth root apex is moved
orthodontically, a greater distance of root apex movement correlates with an
increased risk of EARR.

43

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Abstract (if available)

Abstract The primary purpose of this retrospective, exploratory study was to compare three-dimensional (3D) diagnostics (cone beam computed tomography, CBCT) to two-dimensional (2D) diagnostics (digital panoramic X-ray) in the assessment of root shape and direction of dilacerations in the six maxillary anterior teeth. Panoramic and CBCT images of 50 orthodontic patients were obtained before treatment. Each image was evaluated to assess the shape of the roots and the direction of dilacerations in the six maxillary anterior teeth. In calculating method error, replicate measurements were taken 2 weeks apart on 10 subjects for all variables. An intraclass correlation coefficient (ICC

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Creator Hecht, Ryan S. (author)

Core Title Root shape frequency and direction of dilaceration: a CBCT study

School School of Dentistry

Degree Master of Science

Degree Program Craniofacial Biology

Publication Date 04/05/2013

Defense Date 03/07/2013

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag dentistry,OAI-PMH Harvest

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Advisor Paine, Michael L. (committee chair), Grauer, Dan (committee member), Sameshima, Glenn T. (committee member)

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